1. Technical Field
The present invention relates to stators for rotating electric machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of Related Art
Conventionally, there have been known rotating electric machines which are designed to be used in, for example, motor vehicles and include a rotor and a stator. The rotor is rotatably arranged in the rotating electric machine. The stator includes a stator core and a stator coil. The stator core is disposed in radial opposition to the rotor and has a plurality of slots arranged in a circumferential direction of the stator core. The stator coil is comprised of a plurality of phase windings each of which is mounted on the stator core so as to be received in corresponding slots of the stator core.
In such rotating electric machines as described above, the magnetomotive force of the stator, which functions as an armature, depends on electric current flowing in each phase winding of the stator coil and the number of turns of each phase winding. Therefore, the magnetomotive force of the stator can represent characteristics of the rotating electric machine in relation to the relationship between the torque and rotational speed of the machine. In addition, for each phase winding of the stator coil, the number T of turns of the phase winding can be calculated by the following equation: T=A′/A, where A is the electric current at an input terminal of the phase winding, and A′ is the electric current inputted for each pole. For example, in the case of 4T (i.e., the number T of turns is equal to 4), the torque is kept at a certain high level in a low-speed region, and gradually decreased with increase in the speed. Moreover, in the case of 2T (i.e., the number T of turns is equal to 2), the torque in a low-speed region is about half that in the case of 4T; the width of the low-speed region in which the torque is kept at a certain level is about twice that in the case of 4T; the decrease in the torque in a high-speed region is considerably less than that in the case of 4T; and the torque in the high-speed region is kept at a higher level than that in the case of 4T.
On the other hand, there have been also known segment-type stator coils which are formed by: inserting a plurality of electric conductor segments into slots of a stator core from one axial side of the stator core; and joining each corresponding pair of those ends of the electric conductor segments which protrude from the slots of the stator core on the other axial side of the stator core. In such segment-type stator coils, due to the above manner of forming them, the number of in-slot portions of the electric conductor segments received in each slot of the stator core is generally limited to an even number; and thus the number T of turns of each phase winding is also generally limited to an even number. Consequently, it is generally impossible to obtain a segment-type stator coil which has the characteristics of, for example, 3T between 4T and 2T; with the characteristics of 3T, the torque could be kept moderately high both in a low-speed region and in a high-speed region.
To solve the above problem, Japanese Patent Application Publication No. JP2011045193A (to be referred to as Patent Document 1 hereinafter) discloses a stator coil which has the characteristics of 3T realized by a Δ-Y connection. Specifically, the stator coil is comprised of first and second winding groups. The stator coil is wound around a stator core in a concentrated winding manner so that the phase difference in electrical angle between each corresponding pair of windings of the first and second winding groups is equal to π/6. Moreover, an X-phase winding, a Y-phase winding and a Z-phase winding of the first winding group are Δ-connected to define three terminals therebetween. Further, a U-phase winding, a V-phase winding and a W-phase winding of the second winding group are Y-connected so as to be respectively connected to the three terminals defined between the X-phase, Y-phase and W-phase windings of the first winding group. Consequently, all of the X-phase, Y-phase and W-phase windings of the first winding group and the U-phase, V-phase and W-phase windings of the second winding group together foam a Δ-Y connection (i.e., a combination of a A connection and a Y connection).
Japanese Patent Application Publication No. JP2009112186A (to be referred to as Patent Document 2 hereinafter) discloses a stator coil which is formed of a plurality of continuous electric conductor wires and has the characteristics of 3T realized by connecting two Y connections parallel to each other.
However, the stator coil disclosed in Patent Document 1 involves a problem that circulating current flows within the Δ connection formed of the X-phase, Y-phase and W-phase windings of the first winding group, thereby increasing electrical loss in a high-speed region. On the other hand, the stator coil disclosed in Patent Document 2 involves a problem that circulating current may be generated at portions of the stator coil where the two Y connections are connected parallel to each other, thereby increasing electrical loss.